Don, a stock 49 3600 would not have a 4.10, originally. It would have come with a 4.57, and 5.14 was optional.
In 2010 I made a 300+ mile trip in my 50 3600. With no working speedo I had my wife following me. I told her to hold her speed steady at 55, as much as she could. I did my best to keep a consistent space between us. So I think I averaged 55 for most of it. The old truck did just fine. Loud, but fine. The oil consumption was another matter.
